Hmmm, I think I had read that too and of course one of them was wrong. Google time...
Yeah, now I remember. Depending on the source, Mandela never said a word of it or Williamson had nothing to do with it. I guess I would believe sources which refer to the passage in Return to Love by Williamson, but why is Mandela so often credited?
Some remarks at snopes by readers...
http://message.snopes.com/showthread.php?t=8369